Recent advances in genetic engineering have provided the requisite tools to transform plants to contain foreign genes. It is now possible to produce plants which have unique characteristics of agronomic importance. Certainly, one such advantageous trait is herbicide tolerance. Herbicide-tolerant crop plants could reduce the need for tillage to control weeds, thereby effectively reducing costs to the farmer.
One herbicide which is the subject of much investigation in this regard is N-phosphonomethylglycine. ##STR1## This herbicide is a non-selective, broad spectrum, postemergence herbicide which is registered for use on more than fifty crops. This molecule is an acid, which dissociates in aqueous solution to form phytotoxic anions. Several anionic forms are known. As used herein, the term "glyphosate" refers to the acid and its anions.
Glyphosate inhibits the shikimic acid pathway which provides a precursor for the synthesis of aromatic amino acids. Specifically, glyphosate inhibits the conversion of phosphoenolpyruvate and 3-phosphoshikimic acid to 5-enolpyruvyl-3-phosphoshikimic acid by inhibiting the enzyme 5-enolpyruvyl-3-phosphoshikimate synthase (EPSPS or EPSP synthase).
It has been shown that glyphosate-tolerant plants can be produced by inserting into the genome of the plant the capacity to produce a higher level of EPSP synthase.
The present invention provides a means of enhancing the effectiveness of glyphosate-tolerant plants by producing variant EPSP synthase enzymes which exhibit a lower affinity for glyphosate while maintaining catalytic activity.